An optical pickup for use in a compact disk player, for example, is equipped with an objective lens for focusing a laser beam onto the surface of an optical disk on which information is recorded. Since microscopic pits which are formed in the recording surface of an optical disk at a high density are required to be read, an objective lens of this kind needs a resolution of at least 1 micron and must focus a minute beam spot on the recording surface of the disk accurately. In order to read information correctly, the spherical aberration of the objective lens must be appropriately compensated for. Also, other various conditions are required to be met.
Heretofore, it has been necessary to use a plurality of ground lenses together to form an accurate lens system as mentioned above. The weight of the mirror cylinder increases according to the number of the lenses. Also, a larger load is imposed when the cylinder is moved for a compensating operation. Further, the working distance cannot be made sufficiently long because of the relation to the focal length of the whole lens system.
Recently, it has been proposed to use a single lens having aspherical surfaces formed by pressing on both sides as an objective lens. However, many of conventional pressed lenses are molded out of plastic materials as disclosed in Japanese Patent Laid-Open Nos. 201210/1982 and 76512/1982, for example. These plastic lenses are inferior in durability, resistance to heat, and resistance to chemicals, which arise from the fundamental properties of plastics.
On the other hand, lenses molded out of glass are much superior to plastic lenses in durability, resistance to heat, and resistance to chemicals. However, an aspherical surface is transferred to a glass lens with poor accuracy, because glass materials are treated at much higher temperatures than plastic materials. Also, the higher temperatures shorten the life of the die, as compared with cases of plastic lenses. Since many conventional lenses have aspherical surfaces on both sides, the cost of the die is increased. This greatly affects the cost to form lenses, because dies used to mold glass have a short life. Further, when an aspherical lens is fabricated in practice, the accuracy decreases to thereby produce wave aberration and other aberrations, although an aspherical lens almost free of aberrations can be designed. Especially, for lenses molded out of glass, this tendency is conspicuous for the following reason. When lenses are molded, only very close tolerances are permitted on important factors of lenses, such as axial gap of both surfaces, inclination of surface, and variations in the thickness at the center. When lenses are actually machined, the dimensions are not within the tolerances.